(Trying to help a reader on another forum remember this book - sounds good, I'd like to read it myself!   She says it's NOT Judith McNaught's Kingdom of Dreams.)

I cannot remember the title or author of the book, but here is what I remember of the plot. 

The book takes place in medieval England.  Heroine is a merchant's daughter.  The heroine is rescued by a knight, who is not attracted to her, but to her sister.  The knight and the sister arrange a tryst and at the last minute the sister decides to marry the man her father wants her to marry and sends the heroine off to tell the knight.  The knight mistakes the heroine for the sister in the darkness and "kidnaps" her.  They are forced to marry to avoid a riot or some other trouble.  He treats her badly and there is no love in the marriage.  Enter the hero, who is a knight, but who is somehow forced into disguise as a merchant or some other profession.  He met the heroine through her father b4 the marriage and is attracted to her.  I think his name is something like "Black ????".  Here is where my memory starts to fail me.  There is some danger, like an insurrection and the sister comes to the heroine for protection, they go to an abbey, but the sister runs off and is killed.  The heroine ends up prostituting herself to the sister's husband for some reason and the hero finds out.... it does end up with the knight dying and the hero and heroine marrying, but this might not have been marketed as a romance.

I read this several years ago (more than 3) and it was a hardback book.

If anyone recognizes it and knows the title and/or author I would appreciate any information.  I have been trying to find it for some time now.

It kind of sounds like The Marsh King's Daughter by Elizabeth Chadwick.

Thank you - I passed this suggestion along to the original poster, will see if it bears fruit!